Proximity space links are defined to be short-range, bi-directional, fixed or mobile radio links, generally used to communicate among probes, landers, rovers, orbiting constellations, and orbiting relays. These links are characterized by short time delays, moderate (not weak) signals, and short, independent sessions.

ISO 21459:2006 defines the coding and synchronization sublayer of the Proximity-1 space link protocol. It defines the data unit of this sublayer, i.e. the PLTU. It defines the coding and synchronization mechanisms required for cross support. It also specifies the send and receive side functionality of this sublayer. ISO 21459:2006 does not specify

• individual implementations or products,
• implementation of service interfaces within real systems,
• methods or technologies required to perform the procedures, or
• management activities required to configure and control the protocol.